DESCRIPTION (Adapted from applicant's description): The prevention of osteoporosis is dependent upon maximizing peak bone mass and minimizing subsequent bone loss. During childhood and adolescence, the goal is to increase peak bone mass. The purpose of the study is to investigate the influence of skeletal loading and calcium supplementation with dairy products on bone mineral accretion in the prepubescent skeleton. Fourth grade girls and boys will be selected from two elementary schools. Each school has 4-5 classes per grade with 80-125 children per grade level. The interventions will span two 8-month blocks in a 20-month period and will be conducted in two phases. Phase 1 will be skeletal loading and control groups. Phase 2 will be skeletal loading plus calcium intervention and calcium intervention only groups. The skeletal loading intervention program will include a variety of skipping and box jumping activities designed to stimulate bone formation. A progressive program, to a maximum of fifteen minutes, of jumping activities will be incorporated into the physical education classes that meet twice a week. The dietary groups will be instructed to supplement their diets with dairy products to 1,200 mg/day. The inclusion criteria for the subjects will be: prepubescent, non-smokers, no orthopedic problems, and absence of previous history of metabolic diseases or therapies that may affect bone metabolism. The following data will be gathered at 0, 8, and 20 months: dietary records, calcium & physical activity histories, bone scans, body composition, and Tanner stage. The primary objective is to investigate the effect of an elementary school-based jump rope and calcium intervention program on bone accrual at the lumbar spine, the proximal femur and for the total body in a group of prepubertal boys and girls. A secondary objective is to compare the bone mineral accretion of U.S. children to Canadian children undergoing a similar skeletal loading protocol.